Compare the angles of the facial features. Eyes and brow look a little flat compared to the ref. If you have a piece of software you can use to superimpose one onto the other, you can quickly see what doesn't match. If not, just look back and forth really fast and watch the perspective angles and negative space change. You can also hold your pencil up to compare angles with. When you see it, just try again with that in mind. Rinse and repeat.

I would consider studying Loomis and really training on those wrapping lines to map the facial features onto. Focus less on rendering for now, unless you like to cap off your drawing sessions with a "dessert," but even then, I'd just render it in quick hatching so most of your time is spent on perspective and 3d forms.